Parietal Eye

Some lizards, like iguanas and Bearded
Dragons, have whats called a Parietal Eye.
This 'third-eye' is not a traditional eye, but is a
photosensory organ located on the top of their heads
and is connected to the the lizards pineal
gland.

The Parietal Eye detects changes in
light with its very basic lens and retina. This allows
the lizard to properly thermoregulate their body, and
is why proper light cycles in captivity are important.

Second to thermoregulation, the Parietal Eye also acts
as a defensive measure for lizards,
since an approaching predator will cause the light to
change.